P3A bias setting - diyAudio
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Old 31st January 2010, 07:01 PM   #1
werfi is offline werfi  Finland
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Default P3A bias setting

Sometime (few weeks) ago I finished my first P3A amp. I have designed the pcb myself. I did the testing as described at 60-80W Power Amplifier and all went fine, also adjusting the bias. And the amp also did (and still does) sound good and wasn't noisy.

Now I have finished balanced input on seperate board and mounted all stuff into chassis. I measured voltage drop across resistors and it was almost 100mV. I tried to adjust it back to 50mV, but even with the trim pot at maximum resistance, the voltage was still almost 70mV .

As I said, the pcb is my own design. I have mounted driver transistors (including Q4) onto same heatsink as power transistors, of course they are electrically isolated.
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Old 31st January 2010, 07:58 PM   #2
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Disconnect the balanced input board and try again. Can you set the bias back to 50 mV? If so, then the balanced input board's DC offset could be the culprit.
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Old 31st January 2010, 08:03 PM   #3
werfi is offline werfi  Finland
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I had made those measurements (70mV at max. resistance) with both balanced input board and speaker disconnected.
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Old 31st January 2010, 09:14 PM   #4
mjf is offline mjf  Austria
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hello.
can you measure the offset at the outputs (voltage difference between the two p3a outputs)?
greets
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Old 1st February 2010, 08:35 AM   #5
werfi is offline werfi  Finland
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Hi,

When starting up the amp, the voltage drop across resistors is 0mV. After sometime is climbs up rapidly to 60mV . Maybe it has something to do with transistors heating up?

DC offset at speaker output was 13mV
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Old 1st February 2010, 03:17 PM   #6
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read the construction manual amplifier again .... beyond mistakes that you may be made there is things you missed
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Old 1st February 2010, 03:30 PM   #7
werfi is offline werfi  Finland
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I did read the page again but didn't really figure out what I had missed...
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Old 2nd February 2010, 02:48 AM   #8
jaycee is offline jaycee  United Kingdom
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Where did you locate Q9? Is it near one of the driver transistors Q5 or Q6?

Q4 should not be on the main heatsink - thermal feedback will cause drift. Put it on it's own heatsink.

I would also suggest putting 100uF capacitors on the power supply rails, in parallel with C+ and C-, near the output transistors to help prevent oscillation.

When adjusting bias, do it with no speaker load and with the amp's input shorted to ground.
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Old 2nd February 2010, 11:34 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by werfi View Post
When starting up the amp, the voltage drop across resistors is 0mV. After sometime is climbs up rapidly to 60mV . Maybe it has something to do with transistors heating up?

DC offset at speaker output was 13mV
why is 0mVdc and 60mVdc different from 13mVdc?
What voltages are you measuring?

Q9 (the Vbe multiplier) must be connected to Q5 & Q6 (the drivers). Not to the main heatsink.
All three should be adjacent to each other and connected with a short thick strip of aluminium.
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Last edited by AndrewT; 2nd February 2010 at 11:38 AM.
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Old 2nd February 2010, 01:43 PM   #10
werfi is offline werfi  Finland
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0mV and 60mV are voltage differences between collectors of Q7 and Q8 .
13mV is voltage difference between speaker output and ground.

Quote:
Q9 (the Vbe multiplier) must be connected to Q5 & Q6 (the drivers). Not to the main heatsink.
All three should be adjacent to each other and connected with a short thick strip of aluminium.
I can't understand what you are trying to say. Q9 is in a package that can't be mounted to heatsink.
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